The hydrogen booster idea is pretty interesting...it separates those who understand the laws of thermodynamics, from those who don't
Think about the energy flow. The engine is running on gasoline and air, and it has an alternator that is turned by the engine, and recharges the starting battery and also provides electrical power to run the various things on the vehicle, such as lights, ignition, etc. The alternator therefore is powering the hydrolizer, which separates the hydrogen and oxygen from the water. However, it takes more electrical energy to separate them than you can get back by burning them in the engine!
The whole thing is a scam....although it seems likely that people who put these things on their cars actually do use less fuel, for an interesting side reason. They generally are true believers, and so they fully expect the device to improve their mileage, so they (consciously or subconsciously) change their driving habits, which results in an increase in mileage. In fact, it appears that instructions included with some of the systems for sale state that you have to drive differently with the device. What a happy coincidence!
I have yet to see any published report showing valid data for specific fuel consumption of an engine equipped with one of these things. And a double blind, carefully controlled mileage analysis is unlikely to ever happen....the true believers all know it improves mileage, and the skeptics (who actually might be interested in the results of the experiment) already know it won't improve mileage, so we won't bother to do the experiment because it's a waste of time.